The present invention relates to the field of thermoplastic coatings, particularly suitable for use with wood floors, and more particularly for such wood floors in trucks, vans, cars, railcars and containers, etc.
Currently high-strength hardwood floors are manufactured by cutting hardwood planks into small strips (approximately xc2xexe2x80x3 to 1.15xe2x80x3 or 1.90 cm to 2.9 cm) and subsequently reassembling the strips into 12 in. (approximately 30.5 cm) wide 50 ft. long. (approximately 15.24 m) truck floor planks. The planks are then assembled side by side to form the floor.
The small strips range in length from one foot (30.5 cm) to eight feet (2.44 m). The wide variation in length is due primarily to the removal of defects during manufacture.
These defects are the result of variations in the raw wood supply and they are removed for both structural and cosmetic reasons. Once the strips are sorted, they are glued together using a melamine adhesive (or any suitable glue) that is cured in an RF press. After the assembled plank exits the RF press, it is planed to final thickness. At this phase in the manufacturing operation, cosmetic defects are repaired and the boards are prepped for painting.
The prepared planks are then sent through an automated painting line where they are coated with approximately 7 to 8 mils of water-based paint. After the painting process, the boards are prepared for shipment.
The paint is a water based undercoat and has a low viscosity, so during the curing process some of the paint goes into the wood to be mixed with the fibre of the wood (the paint-wood composite layer). The heat of the curing makes the water evaporate leaving the majority of the solid part of the paint at the surface protecting the wood. At the end of the curing process, the coating of the pure paint protecting the wood is reduced to approximately 3 to 4 mils. Because of the low viscosity and the thin coating protection, the paint does little to hide any imperfection in the wood panel.
Once the pure paint layer is broken at the surface, the paint-wood composite layer can begin to wick moisture into the system via the exposed wood fibers contained in the paint-wood composite. This moisture wicking action serves to hasten the breakdown of the coating.
The primary role of the paint system is to protect the wood, and especially the glue joints, from moisture during the service life of the floor. Based on long term testing in the field, it has been observed that the paint system breaks down after approximately one year in service, with the greatest degradation occurring at the trailer wheel locations. At these key points in the floor, the wood is subjected to an intense spray of water and various road debris that work to weaken the paint and the underlying wood grain, resulting in a quick breakdown of the coating system. Once the paint has broken down, the wood begins to absorb moisture on the roadside of the floor, resulting in warping, uneven swelling stresses, and worst of all, the degradation of the bond in the glue lines of the floor.
Once sufficient moisture has penetrated, the wood grain just inside of the glue lines weakens and the floor can begin to fail. Additionally, once the coating has broken down, water can migrate into the container itself, resulting in product damage. This result is only intensified if the floor warps or swells unevenly. Since the paint system has little or no ability to absorb strain (the ability to stretch) any swelling of the floor leads to immediate cracks in the paint (especially at the glue lines) and further intensifies the problem.
One area of the floor of particular interest is the zone surrounding the end of each hook joint. These areas generally occur 5 to 6 places in each square foot of floor (or 929 cm2), and represent the single largest cause of failure of the paint. Because a small discontinuity exists at each joint and because the current paint system is unable to bridge this discontinuity, the uneven swelling stresses described above are at a maximum at these locations. In typical use, the paint system will develop a small crack at the joints. This crack will begin to allow moisture into the joint, and since the end grain of the wood is fully exposed at this location, the moisture will quickly wick into the stick end causing an uneven, transverse swelling stress. This uneven swelling stress will further crack the paint along the length of the stick, affecting a growing area around each joint. As described above, this swelling and cracking mechanism will work to quickly destroy the paint system at the most critical zones, around the glue joints, and will result in the ultimate destruction of the structural capability of the floor.
It is an object of the present invention to provide an adhesive bonded thermoplastic coating for the underside of a floor as described above that will help solve all of the limitations of the current system, add significant moisture resistance, wear resistance, transverse crack resistance, and make a better looking, more marketable product. In accordance with the invention, this object is achieved with a plastic coated vehicular surface floor having a longitudinal length and a lateral width, said plastic coated vehicular trailer floor comprising:
a. a plurality of wood planks extending longitudinally up to a length substantially equal to the longitudinal length of said floor, each plank having a top surface, a bottom surface opposite said top surface, and a first and second side surfaces extending between said top surface and said bottom surface, where said plank has a width that is less than the lateral width of said floor, each plank being formed by a plurality of boards arranged side-by-side, each board being formed of a plurality of segments joined end-to-end to one another by shaped coupling portions;
b. said plurality of planks being arranged side by side such that at least one of said first and said second side surfaces of each of said planks faces one of said second or first side surfaces of adjacent planks to form the floor, a joint region being formed at locations at which said first side surface faces said second side surface of adjacent planks.
c. each of the plurality of planks of said floor system further comprising a substantially planar polymer layer formed of an un-reinforced polymer, said polymer layer being substantially continuously bonded to said bottom surface of each of the plurality of planks with an adhesive layer and extending unitarily substantially across the width and along the longitudinal length of each plank, the upper surface of each of the plurality of planks being uncoated by the un-reinforced polymer layer, where said plastic polymer layer possesses a modulus of elasticity less than that of the wood.
In a preferred embodiment, the adhesive is an epoxy or reactive hot-melt urethane adhesive (but could be any adhesive including urethane, acrylic, hot melt, polyester, vinyl-ester, reactive hot-melt urethane or other equivalent) and the polymer is a PVC thermoplastic sheet (could be any thermoplastic sheet including PET, Polyurethane, Polyurea, Polyethylene, polypropylene, ABS or other equivalent).